1. Field of the Invention
The present invention relates to a scintillator member (scintillator assembly or scintillator construction) and to a manufacturing method thereof, and a radiation measuring device, and more particularly to light shielding technology of a scintillator member.
2. Description of Related Art
Scintillator members are used in various radiation measuring instruments including a surface contamination measuring instrument, a body surface monitor, and so on. When excited by radiation, a scintillator member emits light, which is then directed onto a light receiving surface of a photomultiplier tube (PMT). In order to detect the light generated in the scintillator with high sensitivity, a light shielding structure or a light tight structure (a dark room) for shielding the scintillator member from external light is provided on both rear and front surface sides of the scintillator member.
With regard to light shielding on the front surface (a radiation entering or incident surface) side of a scintillator member, attenuation of radiation passing through this surface should be considered. In particular, as the range of β rays in the air is small, a light tight film having a certain degree of thickness which is formed on the front surface side of a scintillator member would make blockage and attenuation of radiation on that surface non-negligible, thereby considerably lowering the measuring sensitivity. Thus, only a very thin light tight film can be provided on the front surface side of a scintillator member. Such a thin light tight film, however, has a very low physical strength and is also easy to corrode, when used on its own. Further, when the light tight film is scratched or has pinholes formed by scratching, light enters through such pinholes, making it impossible to detect weak light emission generated in the scintillator member.
In order to overcome the above problems, a technique of providing a plurality of thin light tight films such that they are spaced from each other on a front surface side a scintillator member may be considered, as described in Japanese Patent Laid-Open Publications Nos. 2001-141831 and Hei 3-231187. Each light tight film includes a resin layer and light tight layers formed on both sides of the resin layer. With this structure, even if a light tight layer formed on the top surface of a light tight film provided on the outermost side is damaged, light can be blocked by a light tight layer formed on the rear surface side. Further, when the entire light tight film provided on the outer side is damaged by a large external force, light shielding can be secured by an adjacent light tight film which is spaced from the damaged light tight film toward the inner side. Here, while a protective grille having a lattice shape may be provided on the front surface side of the outermost light tight film as required, there is a possibility that foreign matter or a contaminant may enter through any of the openings of the lattice. If the area of each opening of the protective grille is reduced to thereby enhance the physical protection, the sensitivity for radiation detection would be decreased.
When a light tight film is mounted onto a radiation measuring instrument, it is necessary to dispose the light tight film while pulling the whole film evenly so as to prevent wrinkles from being generated on the film. This task requires technical skill and takes an immense amount of time and labor. Further, the light tight film will easily tear when an excessive force is applied thereto during the disposing operation. Conventionally, these problems are very significant because, in general, a plurality of light tight films are mounted on a radiation measuring instrument.
Japanese Patent Laid-Open Publication No. Hei 7-35869 discloses one example radiation measuring instrument using a scintillator member. Japanese Patent Laid-Open Publication No. Hei 8-248139 describes that a light tight film is attached onto a thin scintillator plate having a large area (see paragraph 0041 and so on). Neither reference, however, describes details of the light tight film or a method of attaching the light tight film. In particular, while it is necessary to dispose an extremely thin light tight film in the case of β ray detection, a treating method specific to such a thin film is not disclosed in any of the references.
Japanese Utility Model Laid-Open Publication No. Sho 62-16486 describes that a light tight film is provided on a surface of a plastic scintillator. The light tight film includes a plastic film in a thin film shape and a deposition layer also in a thin film shape which is formed on a rear or top surface of the plastic film. This reference, however, does not describe how the light tight film in a thin film shape is provided on the plastic scintillator. Japanese Patent Laid-Open Publication No. Hei 5-297145 describes that a scintillator layer and a light tight layer are closely attached. However, the light tight layer is removable and is not fixedly adhered to the scintillator layer.
The present inventors have heretofore repeated various studies and experiments in order to form a thin light tight film having a thickness of approximately 1 to several μm on a surface of a scintillator plate. In an experiment using a vacuum deposition method, a scintillator plate was placed within a vacuum deposition vessel and aluminum was deposited on a surface of the scintillator plate which was being heated. It was found, however, that with this method, the thickness of the film became as much as approximately 20 μm and that when such a thick film was used as a light tight layer, the sensitivity of β ray was decreased considerably. It was also found that, with this method, control of the film thickness was very difficult. On the other hand, while a thin film having a light shielding effect could be formed by an experiment using a sputtering method, it was proved that the strength of the film was very weak and the film was stripped easily when it was rubbed. This method also poses problems that the number of scintillator plates which can be processed per single sputtering operation is limited and that it is difficult to process a large scintillator plate. In addition, while an ion plating method was also studied, with this method, the processing temperature exceeds 100° C., which causes a problem concerning heat resistance of some types of plastic scintillator materials which form the scintillator plate.
Each of the various methods described above requires a large device for forming a light tight film and is also unsuitable for mass processing, resulting in a problem of increased processing cost. In consideration of the background that it is very difficult to form a uniform thin film directly on a scintillator plate, it is currently common to provide a plurality of light tight films which are spaced from each other on a radiation entering surface side of the scintillator plate, as described in Japanese Patent Laid-Open Publication No. 2001-141831 described above. These light tight films, which are formed by applying aluminum deposition onto a very thin plastic film, tear very easily by themselves. Further, each light tight film, which is held in the air, has a very weak structure which is not resistant to an external force. There has therefore been a strong demand for a new light shielding technology in order to increase robustness of a radiation measuring device having a scintillator member.